Date: Thu, 27 Mar 1997 06:12:58 GMT From: Jeffrey Herman To: Multiple recipients of list Subject: Sweep tube linearity From: info-serv@arrl.org Subject: INFO response: TUBEDATA.SUM From: ornitz(Barry Ornitz) Newsgroups: rec.radio.amateur.misc Subject: Sweep tube linearity, etc. (Was: tube finals melt a lot) Date: 5 Jun 91 22:55:45 GMT In an earlier article, I had written about the linearity of sweep tubes and small transmitting tubes. >A real problem with all of the sweep tubes - including the 6146 too, was the >poor linearity of the tubes. I always wondered why no manufacturer ever used >6550's. The power level and voltage ratings of these high linearity tubes >would have been ideal. The Hi-Fi manufacturers were the only companies that >seemed to use them. Perry Scott, AA0ET, replied: >The Radio Handbook contains a table of distortion products for various >sweep tubes. The 6146 is rated at -24 dB of 3rd order IMD, while the >6LQ6 is rated at -18 dB. Some of the linearity problems occur because >of lack of tuned input, which presents a varying load to the exciter in >a grounded grid configuration. Another remedy for nonlinearity is to >use the venerable pi-L to keep the neighbors happy. >I have never heard bad reports on the sweeps I use in the 520 or the >6x6LQ6 linear. If the nonlinearity is just a specsmanship thing, then >I'll be nonlinear. In general, I think sweeps have been much maligned >by people that didn't understand their limitations. They are less >forgiving of design blunders than real transmitting tubes. I am not sure whether Perry is talking about data from the ARRL Handbook or Bill Orr's book. The table in Bill Orr's "Handbook" is from an article he initially published in Ham Radio Magazine (back in 1968, I think). The data given were for Class AB1 service, grid driven. It should be noted that some of the tubes were pushed very hard, often well beyond their normal ratings. In general, the intermodulation distortion of these tubes would be 3 to 5 dB better if they were operated in a cathode-driven mode due to the negative feedback inherent in this operation. However, we are talking about sweep tubes as final amplifier tubes in older SSB exciters, where usually two or more sweep tubes are operated in parallel, and are grid driven with a tuned circuit on the grid and plate. External sweep tube kilowatt amplifiers are a different issue altogether. I have listed some of W6SAI's data on sweep tubes below, along with data on some other older tubes, and some data on a few higher power transmitting tubes and even one power FET. The data is for grid-driven, AB1 operation except where marked with an asterisk which signifies cathode driven operation or with a caret which signifies class AB2 operation. RF LINEAR AMPLIFIER SERVICE FOR SSB AND CW GRID DRIVEN, CLASS AB1 (Except * Cathode Driven,^ AB2) TUBE PLATE SCREEN GRID ZERO SIG MAX SIG MAX SIG PLATE INPUT USEFUL AVG 3-ORD TYPE VOLTS VOLTS VOLTS Ib0 Ib Ic2 LOAD PWR PWR Po DISS IMD V V V ma ma ma ohms W W W dB ------------------------------------------------------------------------------ 6146 600 200 -46 25 103 9 3570 61 41 16 -25 750 200 -51 25 118 7 2825 88 55 28 -22 800 290 -69 30 125 10 3620 100 59 35 -24 800 290 -77 25 180 13 2300 145 91 45 -19 ------------------------------------------------------------------------------ 807 600 300 -34 18 70 8 4300 42 28 12 -23 750 300 -35 15 70 8 5200 53 36 14 -23 ------------------------------------------------------------------------------ 6DQ5 500 150 -46 48 170 17 1800 85 54 27 -28 600 150 -46 48 182 13 1625 91 56 29 -26 700 150 -49 35 182 11 2210 127 78 41 -23 800 180 -67 30 250 13 1710 200 121 70 -19 ------------------------------------------------------------------------------ 6550 680 340 -39 48 140 20 3010 95 67 26 -32 800 290 -33 45 127 15 3920 102 70 29 -30 ------------------------------------------------------------------------------ 6HF5 500 140 -46 40 133 5 1900 67 35 29 -27 800 125 -45 30 197 7 2170 158 100 48 -21 ------------------------------------------------------------------------------ 6JE6 500 125 -44 40 110 4 2300 55 30 24 -26 750 175 -63 27 218 15 1850 163 102 51 -20 ------------------------------------------------------------------------------ 6MJ6/ 750 175 -60 25 215 9 1850 161 102 49 -18 6LQ6 800 200 -69 25 242 13 1850 197 124 60 -18 ------------------------------------------------------------------------------ 4CX300 2K 350 -55 100 250 5 500 300 -27 ------------------------------------------------------------------------------ 8930 2K 350 -63 90 290 30 4000 580 350 -27 ------------------------------------------------------------------------------ *^8877 2.7K triode -8.2 92 740 -- 1820 2000 1085 -40 *^ 3.5K --- -8.2 182 1000 -- 2000 3500 2075 -38 ------------------------------------------------------------------------------ *3CX500A7 4K triode -9 400 1250 -- 1800 5000 3000 -37 * 5K -- -11 400 1450 -- 1800 7250 4000 -35 * 6K -- -12 400 1550 -- 1800 9300 5000 -38 ------------------------------------------------------------------------------ MRF150 50 TMOS 250 3750 -- 333 150PEP -32 A few points should be noted from the data: 1) as the tubes are pushed harder to get higher outputs, the distortion products increase; 2) the 6550 tube shows considerably lower IMD than do comparable sweep tubes at a similar power level (this is expected since the 6550 was designed for low distortion); and 3) _real_ transmitting tubes, i.e. those designed for high power service, show far superior IMD ratings. Unfortunately, Bill did not publish data on the 6JB6A's and 6JS6C's popular in many early SSB rigs (Drake and Yaesu). I included the data on the MRF150 TMOS power FET to show how far solid- state devices have come. It has excellent IMD characteristics. Perry stated that the nonlinearity of the tubes could be handled with some extra tuned circuits. This is only partly true. Higher levels of harmonics, caused by the tube nonlinearity, were usually no problem with the tube rigs because most had excellent Pi-network outputs which required tuning (not broadbanded) and were fairly high-Q. However, the nonlinearity also presents itself as INTERMODULATION distortion, an in-band phenomena which the extra tuned circuits can do nothing about. Consider a simple case of a two-tone signal on 80 meters where two signals are to be amplified by the tube, one at 3.900 MHz and one at 3.901 MHz. Third order IMD will create new signals at 2*f1 - f2 and 2*f2 - f1, or in this case 3.899 MHz and 3.902 MHz. Fifth order IMD will produce signals at 3.898 MHz and 3.903 MHz. These are in-band signals and the extra tuned circuits will do nothing to filter these; however, they might eliminate the problems with the third and fifth harmonics. Excessive IMD can be easily heard - the SSB people call it flat-topping. Your signal gets "mushy" and your bandwidth becomes excessive; other people up and down the band suddenly hear "buckshot" from your signals. IMD increases rapidly as the tubes are driven to saturation. So one important thing to consider if you have one of the older tube rigs is NEVER try to push the power output higher than the manufacturer recommends if you want to remain popular with others on the band (the same thing applies to solid-state rigs too). Nonlinearity is not a specmanship thing, it causes real problems. But do not let this scare you off from using an older rig however. Most of these rigs were capable of good IMD performance by reducing their drive slightly. If you have access to an oscilloscope, it is easy to adjust the rig for the best drive levels and power output consistent with low distortion. The reduction in power will usually be slight, barely enough to show on your contact's S-meter, but the IMD will be much lower. You will also get considerably better tube life. Also keep the tubes cool if you want them to last. Usually all it takes is just turning the microphone gain down a little. I might add that you must be especially careful when using an external speech processor with many of the older rigs. By decreasing the peak to average power ratio, speech processors effectively run up the average power. Many of the older rigs using sweep tubes were not rated for this increase in average power. If you have the manufacturers original specifications, look at the difference between the SSB PEP power levels and the CW/RTTY power levels to see the effect of increased average power. External, multiple sweep tube kilowatt amplifiers, on the other hand, are an atrocity in my opinion. The increase in IMD performance obtained by cathode driven operation is almost always lost because of the use of parallel tubes. Unless perfectly matched tubes are used in these amplifiers, distortion performance will suffer. The only thing going for these amplifiers was the low tube cost (in their day). Look at the 8877 specs if you want to see low distortion. External amplifiers are a different issue than we have been discussing, however, and I prefer to leave the discussions on this to another day. Paul Nix, WB5AGF, asked about RF feedback to reduce distortion. The answer is yes, it works. The best references to this subject are as he suggested, Bill Orr's "Radio Handbook". {While I find W6SAI's discussions on antennas often wrong, Bill KNOWS transmitting tubes, amplifiers, and feedback}. Paul is correct in his statements that feedback lowers stage gains while enhancing linearity. RF negative feedback is an entirely different design problem from negative feedback at audio frequencies. To cover all of the HF amateur bands, the feedback network need cover only slightly more than one decade in frequency. With good audio equipment, the feedback network must work over approximately four decades of frequency range. This is a much tougher problem to do properly. For this reason, many of the audio phreaks have abandoned negative feedback around multiple stages in favor of inherent feedback in individual stages. Collins was about the only amateur radio equipment manufacturer that did much with negative RF feedback in their rigs that I know about. Drake in the L4 amplifier also used the trick of smaller than normal values of the grid bypassing capacitors to obtain some additional negative feedback. Mark Bitterlich, WA3JPY, and Gary Coffman, KE4ZV, also brought up the important point that the matching network on your rig must be tuned at the actual desired power level. You cannot tune up at low power and expect the same matching conditions at high power. One good way to handle this is to tune your rig into a dummy load at the desired power level. Then switch your rig to an antenna tuner to match the antenna. Do not adjust the transmitter at all after switching to the tuner - only adjust the tuner. I have often used an antenna noise bridge preset to 50 ohms to adjust the antenna tuner. In this way, I can tune up without ever putting a signal on the air. Gary mentions the special difficulty in tuning legal-limit amplifiers. One good way to do this involves a pulsed two-tone generator and a scope. You have to use a dummy load for the tests; it is not simple but it works. Finally Perry Scott, AA0ET, suggested that I consider designing a 6550 based project and he also asked the price for the 6550. To start with the second issue, Newark lists the 6550 for around $32. I have seen some audio importers price some 6550's at $15 for Chinese imports. I would tend to stick with the known manufacturers. Now for around $70, I can order the MRF150 transistors. So for new design, I would rather use the single TMOS device to get essentially the same power as a pair of 6550's. With proper care, it will never wear out and I like the lower voltages because of safety. Heat dissipation is easier with the transistor too since radiation cooling and forced convection are seldom needed. As far as retrofitting 6550's into older rigs that used sweep tubes, you will have several problems. First the 6550 has a higher input capacitance than most of the tubes it will replace. The increase in feedback capacitance of the 6550 over most sweep tubes would also usually necessitate changes in the neutralization circuits. Thus you would have to make extensive modifications to the rigs. Finally the physical size of 6550's would mean having to greatly enlarge the final amplifier assemblies in most existing rigs. In conclusion, I would agree with many others - if you can get one of the older tube ham rigs at a reasonable price that works well, do not worry excessively about the tubes and nonlinearity. Crank the power down a little and most of these tube rigs will work fine. Tune quickly though! Ask for HONEST signal reports from others you talk to on the ham bands. Make sure the rig is in proper neutralization and always replace the finals with matched sets (and re-neutralize after any change in finals or driver tubes). As good advice to newcomers to ham radio, find an older ham who may have used these rigs many years ago. They can often remember how to tune these rigs without the long-lost instruction manual, and they can help when you need parts or troubleshooting help too. 73, Barry WA4VZQ